A ghostwriter is usually someone hired to produce a piece of written work, with set terms like deadlines, payment, possibly confidentiality, and other things. Things like memoirs (even some presidents') are ghostwritten by someone who listens to rambling stories and takes notes to produce something readable.

Plagiarism suggests Person B presenting Person A's work as their own without Person A or their intended audience knowing that fact. In this scenario there is no compensation for the claimed work and presumably no communication or cooperation between the writer and plagiarizer.

When you work with a ghostwriter, everyone has agreed to the arrangement up front. And a ghostwriter spends time with you, does interviews, in order to write your story, because they have writing skill and you don't.

Source: Bojack Horseman